This invention relates to vibration isolator systems for limiting the transmission of externally generated vibrational and shock energy to mechanically sensitive components. In particular, the present invention is a vibration isolator system for mounting a housing for an inertial instrument assembly to a support that is subject to shock and vibration. The vibration isolator system uses a single, integral elastomeric member to absorb and dampen shock and vibrational energy.
In certain environments, it is necessary to isolate mechanically sensitive sensor assemblies from shock and vibrational energy. In many applications, this is accomplished by placing the sensor assembly and other necessary elements within some type of container or housing. Resilient, shock and vibration absorbing mounts are frequently used to limit transmission of externally generated vibrational and shock energy into the housing containing the sensor assembly.
The need to isolate a sensor assembly from shock and vibration is particularly acute when the sensor assembly is an inertial sensor assembly (ISA), which is also known as an inertial measurement unit (IMU). An ISA typically includes inertial sensors such as accelerometers and ring laser gyroscopes. Usually there are three accelerometers and three gyroscopes arranged with their input axes in a particular relationship. The sensors are generally rigidly and precisely mounted to a sensor base which, in turn, is precisely mounted within a housing along with related electronics and hardware. Commonly, the housing is in turn mounted to a support or chassis through suspension mounts or vibration isolators. In turn, the chassis is rigidly and precisely mounted to a frame of a vehicle, such as an aircraft.
In operation, the sensors provide inertial data, such as linear and angular acceleration information to a navigational computer onboard the aircraft. The navigational computer processes the data for flight control and/or navigation of the aircraft. For optimum performance, the sensors of the ISA need to provide precise inertial data to the navigational computer. Aircraft maneuvers (i.e., acceleration, changes in pitch, roll and yaw, takeoff and landing), turbulence and engine operation all generate shock and vibrational energy that is conveyed through the aircraft frame to the support for the ISA. This shock and vibrational energy may manifest itself as linear or angular acceleration errors in the inertial data provided by the sensors to the navigational computer. Hence, the need for the shock and vibration isolation of the ISA provided by a vibration isolator.
One such known vibration isolator system 10 for an ISA 12 is illustrated in FIGS. 1 and 2. The ISA 12 includes inertial sensors 14 mounted within a housing 16 defined by a base member 18 and a cover member 20 which are sealed together by a seal ring 22 in a manner known in the art. The inertial sensors 14 are defined by three accelerometers and three ring laser gyroscopes and their associated electronics and hardware as is generally known in the art. An electrical connector 24 mounted in the cover member 20 allows inertial data to be transmitted between the inertial sensors 14 and a navigational computer (not shown) onboard an aircraft.
The base member 18 of the housing 16 includes three mounting lugs 26 (only two of which can be seen in FIG. 1) equally spaced about the circumference of the base member 18. Each mounting lug 26 includes an aperture 28 adapted to receive a threaded fastener 30. The fasteners 30 engage cooperating, threaded openings 32 of inertia ring 34 to rigidly secure the ISA 12 to the inertia ring 34.
As seen best in FIG. 1, the vibration isolator system 10 includes three isolator mounts 36. Each isolator mount 36 includes an outer frame 38 adapted to hold an elastomeric element 40 that provides the isolator mount 36 with its shock and vibration isolation functionality. The elastomeric element 40 is a donut-shaped member which is injection molded onto the outer frame 38 and an inner aperture element 42 simultaneously, using standard injection molding processes generally known in the art. The elastomeric material is a phenyl-methyl vinyl silicone of the form 2FC303A19B37E016F1-11G11 as specified in the American Society for Testing and Materials (ASTM) document ASTM-D2000. Silicone materials of this type are fabricated by numerous manufacturers for a variety of associated applications. The inner aperture element 42 of each elastomeric element 40 is adapted to receive a threaded fastener 44. Each threaded fastener 44 engages a cooperating threaded hole 46 in the inertia ring 34 to secure the elastomeric element 40 of the respective isolator mount 36 to the inertia ring 34 secured to the ISA 12. As seen best in FIG. 1, the isolator mounts 36 are equally spaced about the inertia ring 34. As seen best in FIG. 2, the outer frames 38 of the isolator mounts 36 are secured to a support 48 (shown in dashed lines, and only partially shown relative to one of the isolator mounts 36 for clarity) via threaded fasteners 50. The fasteners 50 pass through apertures 52 of the support 48 to engage threaded openings 54 of the outer frames 38 of the isolator mounts 36.
Though the isolator mounts 36 of the vibration isolator system 10 adequately isolate the ISA 12 from shock and vibration energy conveyed through the support 48, there are some difficulties encountered with the use of multiple discrete isolator mounts. For example, when using multiple discrete isolator mounts it is necessary to match the isolator natural frequencies of the isolator mounts to be used on a selected ISA. In other words, because natural frequency matching is commonly required at the ISA integration level, each individual isolator mount must be tested, segregated, and marked according to its specific natural frequency and amplification factor. The segregated isolator mounts are then packaged as matched sets for installation to a selected ISA. If one isolator mount of the matched set is damaged or lost during the assembly process, the entire matched set must be scraped since unmatched mounts may allow uncompensatable motion of the ISA which will result in inertial data errors.
Another difficulty encountered with the use of multiple discrete isolator mounts results because the discrete mounts are attached at various locations about the ISA. Care must be taken to accurately mount and align the center of gravity (CG) of the ISA on the elastic centers of the isolator mounts. Otherwise CG and elastic center offsets may result in uncompensated rocking and coning motions in the ISA which will manifest themselves in inertial data errors. Therefore, multiple discrete isolator mount systems are difficult to manufacture and hence expensive.
There is a need for improved vibration isolator systems for ISA's. In particular, there is a need for a vibration isolator system that eliminates the need to match the natural frequencies of multiple isolator mounts while providing acceptable shock and vibration isolation of the ISA. In addition, it should be relatively easy to align the elastic center of the vibration isolator system with the CG of the ISA. Lastly, the vibration isolator system should be relatively easy and inexpensive to manufacture.